Research over the preceding grant period has focused increasingly on the use of transgenic and mutant animals for our studies of neoplastic development. The Mutant Rodent Resource, a new shared service, will provide McArdle scientists with the capacity to manipulate the genome of experimental animals by providing ready access to the following technologies: . Generation of transgenic mice and rats by pronuclear microinjection . Generation of chimeric mice by microinjection of embryonic stem cells into host blastocysts . Cryopreservation of mouse sperm